The present invention relates to circuits in the field of analog multipliers and, more specifically, to those types of multiplier circuits whose operation generally does not involve switching on and off of elements such as transistors and/or diodes. In the current technology, each transistor in an analog multiplier""s differential pair where the output is taken, contributes random noise from internal base series resistance, and also each transistor contributes random noise from its collector-to-emitter shot noise current generator.
In the case of a balanced multiplier, such as an LM 1496 or a mixer circuit such as in an LM 1863, both manufactured by National Semiconductor in Santa Clara, Calif., the noise in question comes from the two upper differential pair amplifiers where their bases and collectors are coupled to each other. These upper two differential amplifiers suffer from noise due to their internal base series resistance and collector current shot noise sources. In addition, these xe2x80x9coutputxe2x80x9d amplifiers are sensitive to external noise from one of their inputs, that is, the bases of the upper differential pair amplifiers.
An object of the present invention is to provide an improved signal to noise ratio for a gain control amplifier, analog mixer or multiplier, to lower output noise caused by internal noise and/or to desensitize external noise to an input.
Another object of the invention is to increase the dynamic range of multiplier or mixer circuits known as xe2x80x9cGilbert Quads,xe2x80x9d double balanced analog multiplier circuits, single balanced multiplier circuits, and/or voltage controlled amplifier circuits.
It is another object to maintain or reduce low distortion at the multiplier""s output, or at the voltage controlled amplifier""s output, as the control voltage is varied.
It is yet another object of the invention to not only maintain low distortion, but to further improve on linearity at the output of the voltage controlled amplifier or mixer as the control voltage is varied. This improvement involves paralleled diodes, paralleled diodes in series, serial strings of diodes in parallel, or the like as local feedback elements.
It is also another object of the invention to use similar principles and apply them to single ended amplifiers so that there is an increase in dynamic range with lower distortion when gain control is applied. The single ended controlled amplifier can be used in for example, intermediate frequency (IF) amplifiers in radio receivers.
It is yet another object of the invention to disclose various embodiments of predistortion circuits for improved linearity of multiplier or voltage controlled amplifiers. These predistortion circuits when coupled to a preferred embodiment of this invention comprise a true two port analog multiplier with improved characteristics (i.e., lower noise and/or distortion) over a prior art two port multiplier such as an MC 1495, manufactured by Motorola.
A first embodiment of the invention involves adding a nonlinear element or elements in the local feedback path of at least one differential amplifier to lower noise in a mixer circuit thereof and/or to increase linearity in the mixer circuit. This nonlinear element may be a transistor connected as a diode in series with each differential amplifier""s emitter. In general, it is preferred that the series diodes are connected in a pair or in multiple pairs. Because each of the diodes does have a nominal small signal resistance for a given particular emitter current, the diode pair or pairs serve as a means for reducing the output noise of the differential amplifier(s). By using a series diode or multiple diodes connected to each differential pair emitter, noise caused by series base resistors and/or collector shot noise sources from emitter to collector of each differential pair transistor is reduced.
A diode pair or pairs preferably are used, and output distortion is substantially equal to that of a prior art multiplier circuit when the control voltage at the bases is varied. The reason is that the upper differential pair (output) transistors and the added diodes each have essentially the same (equivalent) bulk emitter series resistance.
It is known that for varying control voltages, there exists a small amount of output distortion in conventional differential pair voltage controlled amplifiers such as the LM 1496 of previous mention. This distortion is caused by equivalent series emitter resistors and/or the internal base series resistors in the upper differential pair transistors. However, as is disclosed by the invention, this distortion can be reduced by adding series-parallel diodes to the emitters. The reason for distortion reduction is because the added series-parallel combinations of diodes dilute the non linearity effect of the series emitter resistors of the upper differential pair transistors.
That is, noise and/or distortion in differential pair voltage controlled amplifiers may be reduced by adding xe2x80x9cNxe2x80x9d sets of xe2x80x9cMxe2x80x9d paralleled diodes in series with the amplifier""s emitters, or by paralleling xe2x80x9cMxe2x80x9d sets of xe2x80x9cNxe2x80x9d serial diode strings in series with the emitters.
The invention contemplates that more noise reduction is achieved by using more than one diode in series with each emitter, as illustrated in FIG. 2A below. In general, use of these diodes allows for the reduction of internal transistor noise at the output of the collector(s), but also for reduction of external noise that may be part of the control voltage, Vcontrol. For example, external noise added to Vcontrol (i.e., via crosstalk of other signals on the circuit board) results in spurious noise and/or modulation noise at the output. Use of the diode feedback elements of the invention changes the transfer function of the control voltage input port and causes a shallower slope in the transfer function of the Vcontrol input versus output signal. Thus, when compared to prior art circuits, the invention provides a circuit of larger control voltage range along with noise reduction, as is illustrated in the discussion of FIG. 2A below.
By using multiple diodes for local feedback, and feeding the signal current through the diodes, a lower noise voltage controlled amplifier is achieved while maintaining low (or original) distortion, as the control voltage is varied. The use of diodes in series with the emitter of the differential amplifier also results in a larger control range and/or more linear voltage range than is provided by prior art circuits.
In a prior art circuit such as shown in FIG. 1B, if perfect transistors were used, there would be no distortion of the signal current (i.e., Isig) when the control voltage is varied. A perfect transistor would have zero ohm internal series base resistance and zero ohm internal series (bulk) emitter resistance. But as previously mentioned, most transistors have finite series internal base and/or internal emitter resistance, which then will cause distortion in a voltage control amplifier when the control voltage is varied.
One distortion component in a typical voltage controlled amplifier occurs when the input signal, part of the xe2x80x9ctailxe2x80x9d current, is set via the control voltage for other than 50% output (i.e., 0 volt across the bases of the upper transistors) or 100% output. This output distortion (i.e., harmonic distortion) arises from the differential amplifier""s transistor internal equivalent emitter series resistance, EESR. Resistance EESR is also determined here by the emitter series bulk resistance plus the total series base resistance divided by the current gain, beta, of the transistor, where beta is considered large. The larger the EESR, the greater the distortion when a control voltage is set for other than 50% or 100% output, for example, when Vcontrol is set for 30% output of the signal from the tail current. Thus the invention described below provides a reduction in the output distortion of a voltage control amplifier which is caused by the resistance EESR. It should be noted that this type of distortion is aggravated further when the operating currents are increased. A circuit such as an LM 1496 which fails to include the emitter series diodes coupled to the upper (output) transistors of the voltage controlled amplifier in accordance with the invention does exhibit this type of distortion.
It should also be noted that in a configuration of the invention with many paralleled diodes in series such as the M pairs paralleled in, for example, FIG. 2F, distortion reduction occurs to the point where even equivalent series emitter resistances of the differential pair transistors become less of a factor in causing distortion.
It follows that in one embodiment of this invention, a way to insure even lower output distortion when the control voltage is set, is to parallel each diode with more diodes. This paralleling of diodes to form a series-parallel connection of diodes reduces the (distortion) effect of series bulk or loss resistance in each diode or differential amplifier transistor. By reducing the effects of series bulk resistance, paralleled diodes become closer to the ideal diode in terms of the exponential Ebers-Moll equation for diode current versus diode voltage. And theoretically if the diodes and/or transistors of the differential pair follow the Ebers-Moll equation, the distortion is zero when the control voltage is varied for bipolar transistors. Of course if a diode is already close to the ideal, paralleling the diodes is not required.
An alternative method of reducing internal and/or external noise may be done at the expense of increasing output distortion by replacing the diodes with resistors. But this method only allows for low distortion when the control voltage is set for a 50% even split of the signal current, or for 100% of signal current. At other proportions, the (external) series emitter resistors actually increase distortion such as that seen for example in a prior art voltage controlled amplifier. However, in this invention, a method of feeding some of the output of the voltage controlled amplifier back to a control input reduces the distortion, thus allowing for a lower noise voltage controlled amplifier or multiplier with low distortion.
It should be noted that in the preferred embodiments, generally a nonlinear element or elements are placed in the transistor or transistors that form the upper output of the multiplier (mixer) or voltage controlled amplifier. But in another embodiment, external series resistors are added to the upper output differential pair as a way of reducing noise. However, in this latter instance, a linearizing circuit is required to reduce the distortion caused by the added emitter series resistance.
Another embodiment of the present invention reduces the distortion in single ended amplifiers by adding at least a diode in series with the emitter while maintaining a wide range of gain control.
Yet another embodiment of the present invention linearizes the control input of the multiplier using multiple diode techniques.